"He sees the threads that join the Universe together and mends them when they break ..."

The Doctor is invited to visit the planet Tigella by its leader, Zastor, but the TARDIS is intercepted by the megalomaniacal xerphyte Meglos. Using the body of a kidnapped Earthling, Meglos transforms himself into a duplicate of the Doctor and seizes the mysterious power source known as the Dodecahedron. Can the Doctor overcome his double and prevent Tigella's destruction? Or will the Gaztak pirates kill him first?